TREASURY STOCK - Narrative (Details) - USD ($) $ / shares in Units, shares in Millions				1 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Feb. 05, 2020	Jun. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 07, 2020
Equity, Class of Treasury Stock [Line Items]
Treasury stock (in shares)	4.9	4.9			4.9		4.9
Treasury stock	$ 234,400,000	$ 235,100,000			$ 234,400,000		$ 235,100,000
Treasury stock, average cost per share (in dollars per share)	$ 47.86	$ 47.86
Stock repurchase program, authorized amount (up to)								$ 225,000,000
Stock repurchased during period (in shares)			1.3	0.6			2.1
Net proceeds from offering of convertible senior notes					$ 0	$ 575,000,000
Accelerated share repurchase program, payment			$ 92,400,000
Accelerated share repurchases, remaining share repurchases expected to be completed, value			$ 100,000,000
Accelerated share repurchases, percentage of expected total repurchased			80.00%
Convertible Debt
Equity, Class of Treasury Stock [Line Items]
Net proceeds from offering of convertible senior notes							$ 100,000,000
Convertible Debt | Certain Purchasers
Equity, Class of Treasury Stock [Line Items]
Net proceeds from offering of convertible senior notes							$ 7,600,000